There is a growing need for large-scale warehousing or collocation of operational computing equipment. Data centers and high-reliability data processing facilities that use large aggregations of computer servers are typically heavy power consumers. Owners and operators of these facilities often desire to use equipment with high power utilization efficiency to reduce operating costs. To this end, UPS systems serving DC power to computer loads, and computer servers using DC power at various voltages, are being considered for their lower losses—due mostly to their reduced number of power conversion stages. For example, there has been much interest lately to power computer racks in data centers with Direct Current (DC) power instead of Alternating Current (AC) power to realize efficiency gains. However, power systems for such configurations present particular problems, i.e. they may involve costly, high amperage equipment and/or have dangerously high fault current levels. The issues of how to do this economically and with a high degree of electrical safety must be addressed.
It is also typical for data centers to house dual-corded computer equipment or servers with “A” and “B” redundant power inputs. UPS systems, therefore, should be configured with “A/B” architecture and be able to accommodate critical loads that “swing” between the “A” and “B” UPS modules. Large facilities, for example, those with critical loads larger than the capacity typically found in single UPS modules, may require multi-module UPS systems. Multi-module UPS systems may be configured in various ways to provide system and/or module redundancy, and to accommodate the A/B nature of power delivery to the critical loads. However, this arrangement often results in large amounts of backup power equipment normally working at low percentages of capacity, becoming nearly full loaded only when in maintenance or failure modes. This often results in higher costs in equipment capitalization and lower electrical efficiency during equipment operation.
Solutions have been developed to handle large critical loads while providing fault isolation among separate, multiple UPS modules (and their loads), and while also providing load sharing among those same modules to make better use of the total system capacity. An example of such a system is the Iso-Parallel UPS shown in U.S. Pat. No. 7,459,803. However, in the case of static UPS systems, it may be necessary to operate such prior iso-parallel configurations such that the power modules operate in on-line fashion (for example, with rectifier and inverter paths engaged, and static bypass switches off). Such an operating configuration, in the case of several types of static UPS, is not the highest efficiency operating mode. Further, such prior iso-parallel configurations supply the critical loads with AC power, and as such the potential efficiency gains of using DC power distribution to the computers is not realized.
Therefore, it would be desirable to provide a parallel bus that would connect multiple static rectifier DC busses and allow DC power to flow from lightly loaded rectifier busses to heavily loaded rectifier busses, and at the same time provide fault isolation among those DC busses. Unfortunately, DC parallel busses can have unacceptably high fault levels, and isolating inductor chokes which reduce fault levels in AC circuits do not work at DC voltages. Furthermore, to make a common parallel bus work in an isolated-parallel fashion it must operate at an AC voltage. If each DC bus has an inverter (or combination inverter/voltage booster) attached that converts the DC voltage to AC at a variable frequency, and also can convert the variable AC back to DC (i.e. perform a bidirectional power conversion), the outputs of those inverters can be attached to reactive chokes and an Isolated-Parallel Bus can be formed.
Furthermore, the inverters, or bidirectional converters (BDCs), need not be three-phase, but can also be single-phase only or two-phase only, and they need not be restricted to an operating frequency of 60 Hz. Indeed, the inverters or bidirectional converters (BDCs) may be operated at an elevated frequency, which would make the Iso-Parallel chokes much smaller and more economical.